While products such as printed boards and electronic parts are usually subjected to processing steps such as soldering, machining and pressing in their way to be made finished products, it almost always occurs during such processing steps that the products become contaminated with flux residual and oil. Such contaminating flux residual and oil must be completely removed away before the products are made finished ones. To this end, it is usually operated to wash the processed products (hereinfater simply called products) with a solvent such as fluorocarbon and 1,1,1-trichloroethane.
However, according to the "Montreal Agreement under Vienna Treaty" of May 1989, the use of fluorocarbon is to be completely abolished by year 2000 A.D., so that the use of fluorocarbon has rapidly come to be suppressed and will become completely abolished eventually. Thus, lately it has grown to be of practice to carry out the washing with use of water which is not causative of environmental pollution.
However, if water remains to be deposited on products after they are washed with water, this gives rise to an insulation failure or corrosion in the cases of printed boards and electronic parts and elements, so that products washed with water should necessarily be dried completely.
Conventionally known drying apparatus include heating driers in which the products are heated to 100.degree. C. or above to evaporate water away, hot air driers in which the products are subjected to flow of hot air and dried, relying on vigorous flow and heat of hot air, and vacuum driers in which the products are dried in vacuum while they are heated at a low temperature.
Using a heating drier, the products are heated to 100.degree. C. or above, and it is likely in the cases of products the heat resistance of which is relatively low that they undergo a thermal damage and/or a lowering of their function or performance. Also, in the case of hot air driers, hot air is blown at a high flow velocity against the products, so that products of a light weight and/or of a fine structure are likely to be blown off to undergo breakage or impairing. In contrast to the above driers, vacuum driers use a heating temperature which is relatively low, and in addition, they do not use a vigorous flow of hot air, so that they are free of the shortcomings of the heating driers and the hot air driers.
However, conventional vacuum driers are only of such a very simple structure as comprising a vacuum container and a heater incorporated therein, so that a large amount of time is required for completely drying a wet product, and where it is required to completely dry a large quantity of products which are continuously supplied, they fail to be satisfactory from the viewpoint of the production efficiency.